1) Field of the Invention
This invention relates to a control system for servo hydraulic pressure in a vehicular automatic transmission, especially to a control system for servo hydraulic pressure adapted to have frictional engagement means engaged or released for changing the gear position of an automatic transmission.
2) Description of the Related Art
An automatic transmission mounted on a vehicle is generally provided with plural planetary gear mechanisms. Upon shifting the speed, shifting elements (gears, carriers, ring gears) of these planetary gear mechanisms are connected in accordance with engagement or release of frictional engagement means to establish a different power flow. The transmission is designed to achieve plural speed stages in this manner. Depending on the arrangement of the planetary gear mechanisms, such a transmission may require simultaneous engagement of frictional elements of plural frictional engagement means upon shifting from a non-running range, in which all the frictional engagement means are released, to a reverse range for driving backward. If it is desired to have such operations done at once upon shifting, a great deal of hydraulic oil must be fed to each frictional engagement means so that the supply of servo pressure may exceed the delivery capacity of a pump as a hydraulic pressure source. This results in prolongation in the time required to raise the servo pressure until the frictional elements are caused to engage, leading to the problem that an unduly large time lag occurs until the planetary gear mechanisms are connected to achieve the reverse range.
For the elimination of such a time lag without increasing the capacity of the hydraulic pressure source, it may be considered effective to have certain frictional engagement means engaged beforehand, said frictional engagement means being some of those engaged in the reverse range and causing no problem even when maintained in an engaged state in the non-running range. An investigation through the prior art from such a viewpoint has revealed that automatic transmissions capable of achieving four speeds by the combination of an underdrive mechanism with planetary gear mechanisms for three forward speeds include those capable of achieving the reverse range by causing a brake of an underdrive mechanism to engage. Such transmissions make use of the construction that the above brake, whose actuation is not particularly required for the actuation of a one-way clutch in a forward drive, is kept engaged even in the non-running range and at the first speed. Apart from the question of whether such a construction was intended to overcome the avove problem, use of such a construction would appear to be effective for its elimination.
According to the above conventional technique, however, shifting to the non-running range during high-speed running of a vehicle causes excessive rotation of the planetary gear mechanisms due to the engagement of the brake. The conventional technique therefore involves the potential problem that the durability of the planetary gear mechanisms may be reduced.